The graphic novel is 12 comics, about 28 pages each. That's 336 pages of dense content to try and convert to a two hour film. It will be a very tall order to get it into a film, but the trailer they've released looks phenomenal. Hopefully they can pull it off!
As a way to generate a little buzz they've released some Watchmen Motion Comics which are video recreations of the actual comic pages. It's pretty badass, I recommend it if you're a comic book fan.